It is customary to drive power looms by an electric motor that is connected to an a.c. power supply or a three-phase power supply network. Preferably, pole-switchable or frequency-controlled motors are used. The main power drive is coupled by power transmission means, such as a belt and pulley drive, to a flywheel mass which is coupled to the main drive shaft of the loom, whereby the electric motor drives the flywheel mass. Prior art systems are so constructed that following the switching-on of the main drive, the flywheel mass is first accelerated by the motor to a predetermined rotational speed (called "rpm" herein for brevity). For starting the loom itself, a clutch-brake unit is used for coupling the flywheel mass to the main drive shaft of the loom, so that the rotating flywheel mass starts the loom from a standstill. The performance characteristic of the clutch the "stiffness" or stability of the motor, and the size of the effective flywheel mass, as well as friction resistances determine a very specific rotational speed or rpm progression characteristic on the one hand for the flywheel mass and on the other hand for the main drive shaft of the loom during the loom start-up process. The rpm of the flywheel mass drops substantially after the clutch is engaged and continues to drop until it matches the rpm of the main drive shaft of the loom that is accelerating from a standstill. During this process, the rpm difference between the main loom drive shaft and the flywheel mass involves the "slipping" of the clutch as it is engaged.
Start-up systems for the looms of the type mentioned above must satisfy special conditions in practice. For example, it is necessary that the loom be completely coupled to its power drive before the first beat up of the reed. It can happen during such a coupling operation that the main loom drive shaft is completely coupled to the power drive before the first reed beat-up, but that the instantaneous rotational speed of the loom is too low at the first reed beat-up. As a result, so-called start-up faults are formed in the fabric. Such start-up faults are formed at places where the inserted weft thread is not beat-up with sufficient force against the beat-up edge of the fabric, resulting in an enlarged spacing between neighboring threads. A series of such enlarged spacings resulting from improperly beat-up weft threads may show up as a stripe-type fabric fault. In order to avoid start-up faults that result from insufficient rotational speed in the start-up phase of the loom, it has been customary heretofore to try to construct the loom drive in such a way that it reaches the desired final instantaneous rotational speed if possible by the time of the first beat-up of the reed.
Such a method for starting up a main drive of a power loom is known from U.S. Pat. No. 4,837,485 (Meroth et al.), which issued to the applicant of the present application. The entire disclosure of U.S. Pat. No. 4,837,485 is incorporated herein by reference. While the system and method of U.S. Pat. No. 4,837,485 are effective for achieving the intended objects thereof, there is still room for further improvement. One shortcoming of the method disclosed therein is that it does not call for precisely determining how much the rated rpm of the main power drive, and thus the rpm of the flywheel mass, must be increased in order for the main loom drive shaft to reach the rated operating rpm by the time of the first beat-up of the reed after the main power drive has coupled with the main loom drive shaft. Instead, the prior method generally involves isolating or disconnecting the main power drive from the power supply network during the coupling operation and then reconnecting it to the network with a greater number of motor poles, for example, in order to select a specific rpm that lies between the starting rpm in the coupling operation and the rated rpm for the weaving operation.